In current infrastructure based networks, a base station (BS), as a centralized controller, is the only access point for mobile devices to the network service. Mobile devices can only communicate with a certain BS via the cellular uplink or downlink. However, enabling device-to-device (D2D) direct communications when mobile devices are close to each other may offer many advantages, which comprise longer battery life, more efficient resource usage, coverage extension, lower interference level and so on. More recently, using the D2D technology in combination with cellular networks is becoming a promising concept. In such D2D enhanced cellular system, D2D direct communications could benefit from the centralized architecture of the cellular networks. On the other hand, the efficiency of the conventional cellular systems can be greatly improved by exploiting the higher channel quality of D2D direct links.